Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for generating a frequency sweep for a seismic source to be deployed next to a sensitive area.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which is information especially helpful in the oil and gas industry. Reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For land applications, sources are mainly vibratory, e.g., a baseplate is driven for a limited time with a desired frequency selected from a given frequency range for generating seismic waves that propagate through the earth. To cover the entire frequency range, a frequency sweep is applied by a controller to a seismic source, i.e., it sweeps the frequency range so that all frequencies in the range are applied to the baseplate. Vibratory sources may include hydraulically-powered sources or piezoelectric or magnetostrictive material.
Alternatively, impulsive sources may be used for generating the acoustic waves. An impulsive source may include explosive material that is detonated to create seismic waves.
A general problem associated with the use of seismic energy sources for land seismic surveys, vibratory or explosive, is that these sources generate ground motion which can damage nearby infrastructure, such as roads, buildings, pipelines, etc. Thus, various countries have imposed different regulations for limiting the level of ground motion that seismic sources are allowed to generate in the vicinity of infrastructures. For example, FIG. 1 illustrates some regulations used around the world in which ground motion is measured as a particle velocity on three mutually perpendicular components, usually the vertical, radial and transverse directions. A PPV (i.e., peak particle velocity) value is computed from particle velocity measurements. It is the PPV value that is typically capped as a function of frequency for different kinds of buildings and vibrations. The term PPV is defined as the peak particle velocity estimate that was computed using particle velocity measurements from one or more components. A suffix is added to term PPV if the measurement was made using only a single component, for example, PPV_vert is the vertical component, PPV_rad is the radial component. If it is a vector sum of component peak amplitudes, the term is PVS, (i.e. Peak Vector Sum). The term CPPV (i.e., capped peak particle velocity) is used to represent the PPV threshold not to be exceeded and this threshold is set by the company conducting the seismic survey so that the company complies with local regulations. The term CPPV is herein used to apply to whatever motion threshold cannot be exceeded and can be based upon PVS, PPV_vert or a combination of component values to form a predetermined threshold not to be exceeded. Note that a company may choose to apply more restrictive CPPV values based on external recommendations to ensure no damage is inflicted on any neighboring infrastructure than what a governmental agency might specify.
Thus, the company performing the seismic survey may install sensors around infrastructures in or close to the seismic survey and monitor the data these sensors record to ensure they are compliant with local regulations.
Additionally, the company performing the seismic survey may choose to enforce buffer zones according to minimum safety distances established in the industry, such as those published by the International Association of Geophysical Contractors. However, safety buffers reduce the quality of acquired seismic data by reducing the fold of coverage. An additional common practice is to enforce low-drive buffer zones to reduce the force of the vibration. This allows recording seismic data over a larger area thereby helping to preserve adequate spatial sampling, but at the expense of a lower signal-to-noise ratio due to the lower signal amplitude.
Various solutions have been proposed over time to prevent recorded seismic data degradation while preserving the integrity of the seismic survey area's infrastructure.
Rappin et al. (“Determination of safety distances and source monitoring during land seismic acquisition,” Chapter 8, Pages 41-45) proposes a two-step method to adjust safety distances. First, a PPV versus offset is measured on the seismic site. Then, the PPV versus offset curve is fitted and interpolated. Second, a safety distance is deduced from this curve, matching the enforced CPPV, (i.e. PPV threshold), with a safety margin. An optional recommended third step is to monitor each structure presenting a risk during the seismic acquisition.
Favret and Genaud (U.S. Pat. No. 6,152,256, the entire content of which is included herein by reference), proposes to modulate the force of the vibration as a function of frequency to match a PPV versus frequency curve. The curve can be empirical or theoretical. Compared to the low-drive buffer zone approach, this approach allows preservation of frequencies that do not threaten structures. However, this and other existing methods do not maintain the original target power spectral density, and, thus, generate noisier seismic data in sensitive areas.
All these traditional approaches are still limited in the sense that the full potential of the seismic source is not tapped for fear of damaging existing structures. Therefore, there is a need for a method that can adjust a seismic source force as a function of frequency to take into account the CPPV but also to modulate the source sweep rate to maintain the emitted spectral density for vibrator source points swept in sensitive areas. Note that the instantaneous sweep rate is the time derivative of the sweep frequency versus time function.